1. Field of the Invention
This invention relates to an internal combustion engine including a plurality of cylinders split into first and second groups and operating in a split cylinder mode under low load conditions where the first group of cylinders are held operative and the second group of cylinders are held suspended and, more particularly, to such an internal combustion engine of cylinders can operate wherein the second group with suppressed pumping loss during a split cylinder mode of operation.
2. Description of the Prior Art
It is generally known that internal combustion engines exhibit higher fuel combustion and thus higher fuel economy when running under higher load conditions. In view of this fact, split type internal combustion engines have already been proposed as automotive vehicle engines or the like subject to frequent engine load variations. Such split type internal combustion engines comprise a plurality of cylinders split into first and second groups, an intake passage provided therein with a throttle valve and bifurcated downstream of the throttle valve into a first branch leading to the first group of cylinders and a second branch leading to the second group of cylinders, an air stop valve provided at the entrance of the second branch for opening and closing the second branch, and control means responsive to engine low load conditions for closing the air stop valve to prevent fresh air from flowing into the second group of cylinders and for cutting off the supply of fuel into the second group of cylinders so as to place the engine in its split cylinder mode of operation. As a result, the other operative cylinders can operate with high loads, which results in high fuel economy.
For higher fuel economy, it is desirable to suppress pumping loss in the second group of cylinders during a split cylinder mode of operation. For this purpose, an attempt has been made to simultaneously close the intake and exhaust valves of the second group of cylinders during a split cylinder mode of operation. However, it requires a sophisticated mechanism for simultaneously closing the intake and exhaust valves of the second group of cylinders, it spoils the stability of rotation of the crank shaft since the pressure developed in the combustion chambers of the second group of cylinders varies in synchronism with the angle of rotation of the crank shaft, and it causes a great amount of oil to be sucked into the combustion chambers and soil them if the engine is placed in its split cylinder mode when the pressure in the combustion chambers is low.
Another attempt has also been made to re-introduce exhaust gases into the second group of cylinders during a split cylinder mode of operation. The effect to suppress pumping loss increases as the amount of exhaust gases re-introduced increases. However, it requires a large diameter exhaust gas re-introduction (EGR) passage with low flow friction and thus a large EGR valve in order to re-introduce, into the second group of cylinders, as great an amount of exhaust gases as possible.